Recently, densities of optical discs have been further increased. For example, a phase change optical disc, on which preformed grooves have a pitch of 0.32 μm, information tracks also have a track pitch of 0.32 μm and a coating layer to protect its information storage layer has a thickness of 0.1 mm, is known as a typical optical disc with an increased density. To read and/or write data from/on such an optical disc, the optical head of an optical disc drive may include a light source with a wavelength of 405 nm and an objective lens with a numerical aperture NA of 0.85, for example.
A high-density optical disc has such a narrow information track pitch that the optical head needs to perform a tracking control operation with very high precision. Accordingly, once a deviation has occurred in a tracking error signal due to the shift of the objective lens or the tilt of the optical disc, the tracking control operation can no longer be performed with the required precision, thus causing some problems. As used herein, the “deviation” of the tracking error signal refers to a gap (or distance) between the beam spot location and the center of the information track when the tracking error signal crosses the zero level.
Also, data is written on a phase change optical disc by changing the reflectance. The phase change optical disc has a narrow track pitch between adjacent information tracks. Accordingly, if the adjacent information tracks have mutually different reflectances, then the tracking error signal could not be detected accurately due to the difference. Furthermore, due to an error occurring during the manufacturing process of optical discs, the information track pitch might be variable from one location to another. In that case, the tracking error signal could not be detected accurately, either. Then, the tracking control operation could not be carried out with sufficient precision on a high-density optical disc. For that reason, efforts for detecting the tracking error signal as accurately as possible have been carried on.
For example, PCT International Application Publication No. WO 97/15923 discloses a technique of reducing the deviation that could occur in a tracking error signal due to the shift of an objective lens and the tilt of an optical disc. Hereinafter, a configuration for realizing the technique disclosed in this document will be described.
FIG. 1(a) shows a configuration for a conventional optical head 100. The optical head 100 includes: a light source 101; a lens 102 for transforming the light emitted from the light source 101 into luminous flux of parallel rays; half mirrors 103 and 106 for reflecting a portion of incoming light and transmitting another portion of the light; an objective lens 104 for focusing the light; a focus control section 107 for performing a focus control operation by detecting the luminous flux that has been reflected from an optical disc 105; and a light receiving section 108 for receiving the luminous flux, reflected from the optical disc 105, at a plurality of light receiving areas.
The optical head 100 operates in the following manner. Specifically, the light emitted from the light source 101 is focused on the optical disc 105 by way of the lens 102, half mirror 103 and objective lens 104. A portion of the luminous flux of reflected rays, which has been reflected from the optical disc 105, is further reflected by the half mirror 106 toward the focus control section 107. The rest of the reflected luminous flux is transmitted through the half mirror 106 to enter the light receiving section 108. The focus control section 107 detects a focus error signal based on the received signal and controls the distance between the objective lens 104 and the optical disc 105 with the detected signal such that the light emitted from the light source 101 is focused on the optical disc 105 with a desired precision. On the other hand, the signal received at the light receiving section 108 is used to generate a tracking error signal. The light is focused on an information track, which extends in the direction coming out of the paper.
FIG. 1(b) shows the cross section of the luminous flux received at the light receiving section 108. The cross section 110 of the luminous flux is split into two by a division line 109a, which extends parallel to the information tracks, and further split by two more division lines 109b and 109c, which cross the division line 109a at right angles. In this manner, the cross section of the luminous flux is divided into eight areas 108a through 108h. As the optical disc 105 rotates, the beam spot of the reflected light apparently moves from the area 108a toward the area 108g, for example. On the other hand, the area 108i is a light shielding area, which cuts off the incident light. The light rays falling on these areas 108a through 108h are detected as signals representing their respective quantities of light. If the signals detected by these areas 108a through 108h are identified by their reference numerals, the tracking error signal TE is given by:TE=(108c+108e−108d−108f)−k(108a+108g−108b−108h)  (1)where k is a correction factor. If the objective lens 104 is moved perpendicularly to the information tracks, then the cross section 110 of the luminous flux shifts perpendicularly to the division line 109a on the light receiving section 108. As a result, an offset is produced in a so-called push-pull signal (108c+108e−108d−108f). This offset is corrected with the signal (k(108a+108g−108b−108h)). The correction factor k is defined such that the offset of the tracking error signal decreases as the objective lens 104 moves.
It should be noted that the “deviation” and “offset” of the tracking error signal are two different notions. Specifically, the “deviation” of the tracking error signal changes incessantly according to the positional relationship between the focal point of the light beam and the information storage layer and the reflectance of the information storage layer. Accordingly, the deviation cannot be minimized uniformly by using a factor such as the correction factor described above. On the other hand, the “offset” of the tracking error signal is produced steadily in the overall signal due to the shift of the objective lens described above, for example, and can be reduced uniformly with the correction factor.
FIG. 2 schematically shows the light intensity distribution of the luminous flux 110 that has been reflected from the tilted optical disc 105. As used herein, “tilting” means tilting a normal, which is defined perpendicularly to the optical disc 105, parallel to the paper in FIG. 1(a). When the optical disc 105 tilts, the light diffracts on the information track, thus making the light intensity distribution of the reflected light non-uniform. In FIG. 2, the hatched areas 110-1 represent the areas in which zero-order and first-order components of the luminous flux diffracted by the information track are superposed one upon the other. The remaining area 110-2 of the luminous flux 110, other than the hatched areas 110-1, represents an area receiving only the zero-order diffracted light, i.e., where no diffracted light components are superposed. As can be seen from FIG. 2, the hatched areas 110-1 are asymmetric with respect to the division line 109a. 
When the light intensity distribution is asymmetric, the light receiving section 108 cannot judge whether that state was caused by the tilt of the optical disc 105 or by a bad tracking state. If a tracking error signal is generated from such reflected light, then the tracking error signal will represent the bad tracking state all the way and therefore have a low signal precision. For that reason, the light shielding area 108i is provided as shown in FIG. 1(b) such that the portion of the light corresponding to the significantly asymmetric portion of the intensity distribution is not converted into a signal (i.e., not used to generate or detect a tracking error signal). In this manner, the detection errors of the tracking error signal can be reduced.
There are some optical discs on which address information and other identification information is pre-recorded as wobbles of information tracks (i.e., by way of a wobble signal). FIG. 3 is a partial enlarged view of wobbled information tracks 105-1 through 105-3 on the optical disc 105. These information tracks 105-1 through 105-3 have respective wobbles representing the information recorded there. The wobbles of the three information tracks 105-1 through 105-3 change independently of each other. The technique of detecting such a wobble signal with a push-pull signal is disclosed in Japanese Laid-Open Publication No. 7-14172, for example.
The tracking error signal to be obtained from the light reflected from the high-density optical disc 105 will be described by reference to the results of numerical simulations. Those simulations were done with the conventional optical head 100 shown in FIG. 1 under the conditions that the light had a wavelength of 405 nm, the information tracks had a track pitch of 0.32 μm and a groove depth that was one-twelfth of the wavelength, the NA was 0.85, and the objective lens had a focal length of 2 mm. It should be noted that the information track groove is a recessed groove defined between two raised information tracks. The information tracks and information track grooves are defined herein as such. However, when viewed from over the other side of the optical disc 105, the tracks and track grooves will be recessed and raised, respectively. Thus, the following description will be mainly focused on the “information tracks”.
Hereinafter, it will be described how the tracking error signal should be affected in respective situations where the objective lens has shifted, where the optical disc is tilted, where the pitch between the information tracks is variable, and where adjacent information tracks have mutually different reflectances.
(1) Offset of Tracking Error Signal Due to the Shift of Objective Lens:
FIG. 4 schematically shows the light intensity distribution of a luminous flux that was reflected from the high-density optical disc and detected by the light receiving section. The hatched areas represent areas where the zero-order and first-order components of the luminous flux, diffracted by the information track grooves of the optical disc 105, were superposed one upon the other. The other non-hatched area consisted essentially of the zero-order diffracted light only (i.e., the area where no diffracted light components were superposed). Compared with the light intensity distribution shown in FIG. 1(b) or FIG. 2, the gap L between the two hatched areas is wider and the area consisting essentially of the zero-order diffracted light is broader in the light intensity distribution shown in FIG. 4. This is because the high-density optical disc has a narrower pitch between the information tracks.
In such a distribution, the push-pull signal has a low degree of modulation, and therefore, even if the objective lens 104 has been shifted just slightly, a non-negligible offset will be produced in the signal. Such a luminous flux is received at the respective divided areas shown in FIG. 4. The gap between the two division lines 109b and 109c is approximately equal to the maximum width of the diffracted light superposed areas (i.e., the hatched areas shown in FIG. 4) as measured along the division line 109a. It should be noted that the light shielding area 108i shown in FIG. 1(b) is not provided because the offset correction is the only concern here.
FIG. 5 shows a relationship between the shift of the objective lens 104 and the symmetry of the tracking error signal. The abscissa represents the shift of the objective lens while the ordinate represents the symmetry of the tracking error signal. The symmetry is obtained herein as the ratio of the offset voltage to the voltage amplitude. It can be seen that while the shift of the objective lens falls within the range of ±0.3 mm, good symmetry is maintained and the offset is corrected sufficiently.
(2) Non-Uniform Information Track Pitch
If the pitch between information tracks is non-uniform, then some pair of adjacent information tracks at one location on the optical disc 105 may be relatively close to each other but another pair of adjacent information tracks at another location on the optical disc 105 may be relatively distant from each other. FIG. 6 shows a model on which the leftmost one of three information tracks may be displaced. On this model, the waveform of the tracking error signal was simulated.
FIGS. 7(a) and 7(b) show the waveforms of the tracking error signals obtained by displacing the leftmost information track. In this case, the information track was displaced by ±20 nm. FIG. 7(a) shows the waveforms of push-pull signals with no offset corrected. In FIG. 7(a), the beam spot location (i.e., the focal point of the light beam) of zero is the center of the reference information track and corresponds to the center of the central track shown in FIG. 6 (which will be referred to herein as the “center track”). In the range where the beam spot locations are negative, signals representing the leftmost information track to be displaced in FIG. 6 appear. On the other hand, in the range where the beam spot locations are positive, signals representing the rightmost information track in FIG. 6 appear.
The three signal waveforms 114, 115 and 116 shown in FIG. 7(a) are signals obtained by displacing the leftmost information track in three stages. More specifically, the waveform 115 is obtained by arranging the leftmost information track at the originally designed location; the waveform 114 is obtained by displacing the leftmost information track by 20 nm toward the center track; and the waveform 116 is obtained by displacing the leftmost information track by 20 nm away from the center track. It should be noted that the rightmost track is supposed to be arranged just as originally designed. In FIG. 7(a), a significant offset variation is observed in the vicinity of each information track displaced from the original location (i.e., near the left-hand-side peak of the waveform), which shows the effects of the non-uniform information track pitch.
FIG. 7(b) shows the waveforms of tracking error signals of which the offsets are corrected. The offsets are corrected with an objective lens position signal. As in FIG. 7(a), the three waveforms 117, 118 and 119 shown in FIG. 7(b) are signals obtained by displacing the leftmost information track in three stages. More specifically, the waveform 118 is obtained by arranging the leftmost information track at the originally designed location; the waveform 117 is obtained by displacing the leftmost information track by 20 nm toward the center track; and the waveform 119 is obtained by displacing the leftmost information track by 20 nm away from the center track. Compared with FIG. 7(a), the effects of the offset variation on the signal waveforms are reduced significantly in FIG. 7(b). This means that the offset variation caused by the non-uniformity of information track pitches is corrected by the offset correcting means described above. It should be noted that the correction factor used in the calculations for FIG. 7(b) has the same value as the correction factor for correcting the offset of the objective lens as described above.
(3) Tilt of the Optical Disc:
Next, it will be considered whether or not the deviation of the tracking error signal due to the tilt of the optical disc 105 can be reduced when the light shielding area 108i (see FIG. 1(b)) is provided for the light receiving section 108. It should be noted that the “deviation” of the tracking error signal refers to the gap (or distance) between the beam spot location and the center of the information track when the tracking error signal crosses the zero level as described above.
FIG. 8 shows how the waveform of the tracking error signal changes when the width of the light shielding area 108i is changed in the direction in which the division line 109a (see FIG. 1(b)) extends. The abscissa represents the beam spot location, which is defined with respect to the center of the information track as the origin, while the ordinate represents the signal level. The width of the light shielding area 108i was changed in three steps, thereby obtaining three waveforms 111, 112 and 113. Specifically, the waveform 111 was obtained when the width of the light shielding area 108i was zero (i.e., when no light shielding area 108i was provided), the waveform 112 was obtained when the width of the light shielding area 108i was 20% of the diameter of the luminous flux, and the waveform 113 was obtained when the width of the light shielding area 108i was 35% of the diameter of the luminous flux. The coating layer of the optical disc 105 had a thickness of 100 μm and the disc had a tilt of 0.6 degrees.
As can be seen from FIG. 8, at the zero beam spot location, the signal levels should have been equal to zero but the actual signal levels decreased below the zero level and the magnitudes of the deviations were in the order of the waveforms 111, 112 and 113. That is to say, the greater the width of the light shielding area 108i, the greater the deviation of the tracking error signal. Consequently, as for high-density optical discs to be developed and put on the market in the near future, the deviation of the tracking error signal cannot be reduced by the conventional technique.
(4) Offset Produced in the Tracking Error Signal on the Boundary Between Information Tracks with Different Reflectances:
FIG. 9 shows the cross sections of three information tracks. Among the tracks shown in FIG. 9, the center track and another information track on the right-hand side are supposed to have a reflectance of 1 and be unrecorded tracks. On the other hand, the other hatched information track on the left-hand side is supposed to have a reflectance of 0.5 and be a recorded track. The optical disc 105 is supposed to have multiple sets of these tracks that are arranged periodically.
FIG. 10(a) shows the waveforms of push-pull signals of which the offsets are not corrected. Among the three signal waveforms shown in FIG. 10(a), the bold curve represents the waveform of a push-pull signal at the focal point of the beam spot, the solid curve represents the waveform of a push-pull signal when the focal point of the beam spot is shifted by +0.2 μm, and the dashed curve represents the waveform of a push-pull signal when the focal point of the beam spot is shifted by −0.2 μm. The focal point of the beam spot is “shifted” perpendicularly to the optical disc 105, i.e., in the direction in which the light beam is radiated toward the optical disc 105. In this case, the positive direction refers to shifting the focal point toward the optical head as viewed from the optical disc 105, while the negative direction refers to shifting the focal point in the opposite direction. In these waveforms, a significant offset variation due to the difference in reflectance between adjacent information tracks and the change of that offset variation with the focus shift of the beam spot are recognized.
Thus, the waveforms of offset-corrected tracking error signals were calculated using the same correction factor as that used to correct the offset resulting from the shift of the objective lens. FIG. 10(b) shows the waveforms of tracking error signals of which the offsets are corrected. Among the three signal waveforms shown in FIG. 10(b), the bold curve represents the waveform of a push-pull signal at the focal point of the beam spot, the solid curve represents the waveform of a push-pull signal when the focal point of the beam spot is shifted by +0.2 μm, and the dashed curve represents the waveform of a push-pull signal when the focal point of the beam spot is shifted by −0.2 μm. It can be seen that the offset variations of the three waveforms shown in FIG. 10(b) are much less remarkable than those shown in FIG. 10(a).
However, around the beam spot locations of 0 μm and +0.32 μm, the offset variation changes with the focal point of the beam spot and the three waveforms cross the zero level at mutually different locations. In addition, the magnitude of change of the offset variation with the focal point of the beam spot is not constant among the three information tracks. Accordingly, even if the correction factor for the offset correction is changed, the deviation of the tracking error cannot always be reduced for every information track.
To sum up, the above-mentioned combination of conditions for a high-density optical disc has no effects of reducing the deviation of the tracking error signal resulting from the tilt of the optical disc as described for (3) and cannot correct sufficiently the offset variation produced in the tracking error signal on the boundary between information tracks with different reflectances (or the offset variation caused by the focus shift of the beam spot, in particular) as described for (4).
Furthermore, in an optical disc with wobbled information tracks (see FIG. 3), the gap between the central information track and its adjacent information track changes with this wobble. As already described with reference to FIGS. 7(a) and 7(b), the push-pull signal is easily affected by the adjacent information track. The wobble signal of one information track may be represented by the distance from its adjacent information track. Accordingly, if the wobble signal is detected with the push-pull signal to be easily affected by the adjacent information track, then crosstalk will increase between the wobble signals of the two adjacent information tracks.